FIG. 5 shows an example of a conventional structure of a wheel-driving bearing unit in which a wheel-supporting rolling bearing unit disclosed in Patent Literature 1 is incorporated. The wheel-driving bearing unit shown in FIG. 5 is formed by combining a wheel-supporting rolling bearing unit 1 and an outer ring 2 for a constant velocity joint. The wheel-supporting rolling bearing unit 1 includes an outer ring 3, a hub 4, and a plurality of rolling elements (in the example shown, balls) 5 and 5.
The outer ring 3 has a stationary-side flange 6 formed on an outer circumferential surface thereof, and a plurality of rows of outer ring tracks 7a and 7b formed on an inner circumferential surface thereof.
The hub 4 is formed by combining a hub main body 8 and an inner ring 9. The hub main body 8 has a rotation-side flange 10 formed on a portion of the outer circumferential surface of hub main body 8 close to one end in an axial direction. In addition, the hub main body 8 has an inner ring track 11a formed on an intermediate section of the outer circumferential surface in the axial direction, at one side in the axial direction. In addition, the hub main body 8 has a small diameter step section 12 formed on the other end portion of the outer circumferential surface in the axial direction, and a center hole 13 formed in a central section thereof.
Further, in the specification and claims, “one side” in the axial direction refers to an outside in a widthwise direction of a vehicle in a state in which a bearing is attached to an automobile. That is, a left side of FIG. 5 and a lower side of FIG. 6 are referred to as “one side” in the axial direction, and on the other hand, a right side of FIG. 5 and an upper side of FIG. 6, which are central sides in the vehicle when a bearing is attached to an automobile are referred to as “the other side” in the axial direction.
A small diameter section 14 through which a rod section 16 of a bolt 15 serving as a coupling member can be inserted via a predetermined guiding clearance is formed in one end portion of the center hole 13 in the axial direction.
The inner ring 9 has an inner ring track 11b formed on the other side of the outer circumferential surface in the axial direction, and is fastened and fixed onto the small diameter step section 12 of the hub main body 8.
The rolling elements 5 and 5 are rollably installed between both of the outer ring tracks 7a and 7b and both of the inner ring tracks 11a and 11b such that the plurality of rolling elements are disposed in each of the rows.
In this state, a caulking section 20 is formed by plastically deforming a portion of a cylindrical section 19, formed on the other end portion of the hub main body 8 in the axial direction, protruding from the other end opening of the inner ring 9 in the axial direction outward in the radial direction. An appropriate preload is applied to the rolling elements 5 and 5 by pressing the other end surface of the inner ring 9 in the axial direction with the caulking section 20.
A hub-side face spline 21 that is a concavo-convex section in the circumferential direction is formed on the other end surface of the caulking section 20 in the axial direction throughout the circumference.
Further, in the case of the example shown, a tooth tip surface of the hub-side face spline 21 is made as a plane perpendicular with respect to a central axis of the hub main body 8.
The outer ring 2 for a constant velocity joint has a cup-shaped mouth section 22, an end wall section 23 that is a bottom section of the mouth section 22, and a cylindrical shaft section 24 extending from a central section of the end wall section 23 toward one side in the axial direction. A center hole of the shaft section 24 is a screw hole 25.
A joint-side face spline 26 that is a concavo-convex section in the circumferential direction is formed on a portion close to an outer circumference of one end surface of the end wall section 23 in the axial direction throughout the circumference.
Further, in the case of the example shown, a tooth tip surface of the joint-side face spline 26 is made as a plane perpendicular to the central axis of the outer ring 2 for a constant velocity joint. In addition, the number of teeth of the joint-side face spline 26 is the same as the number of teeth of the hub-side face spline 21.
Then, in a state in which central axes of the hub main body 8 and the outer ring 2 for a constant velocity joint coincide with each other, as the face splines 21 and 26 of both of the hub-side and the joint-side are meshed with each other, transmission of a rotating force between the hub main body 8 and the outer ring 2 for a constant velocity joint are realized. In addition, in this state, the rod section 16 of the bolt 15 is inserted through the small diameter section 14 of the center hole 13 of the hub main body 8 from one side in the axial direction and a male screw portion 17, formed on a tip portion of the rod section 16, is screwed into the screw hole 25 for further fastening. Accordingly, in a state in which the hub main body 8 is disposed between a head portion 18 of the bolt 15 and the outer ring 2 for a constant velocity joint, the hub main body 8 and the outer ring 2 for a constant velocity joint are coupled and fixed.
When the wheel-driving bearing unit configured as above is assembled in the vehicle, a rotary member for brake such as a wheel (a driving wheel), a disk, and so on, are supported by and fixed to the rotation-side flange 10 of the hub main body 8 while the stationary-side flange 6 of the outer ring 3 is coupled and fixed to a suspension system. In addition, a tip portion of a drive shaft (not shown), rotated and driven by an engine via a transmission, is spline-coupled to an inside of an inner ring 27 for a constant velocity joint installed inside of the outer ring 2 for a constant velocity joint. During traveling of an automobile, rotation of the inner ring 27 for a constant velocity joint is transmitted to the outer ring 2 for a constant velocity joint and the hub main body 8 via a plurality of balls 28 to rotate and drive the wheel.
When the wheel-supporting rolling bearing unit 1 that constitutes the wheel-driving bearing unit configured as above is assembled, first, the outer ring 3 is disposed around the hub main body 8, and the rolling elements 5 and 5 are installed in a state held by a holder 29a disposed at one side in the axial direction in between the outer ring track 7a at the outer ring tracks 7a and 7b of one side in the axial direction and the inner ring track 11a at the one side in the axial direction. Next, the rolling elements 5 and 5 are installed in a state held by a holder 29b disposed at the other side in the axial direction in the surroundings of the inner ring track 11b of the other side in the axial direction formed on the outer circumferential surface of the inner ring 9, and in this state, the inner ring 9 is fastened and fitted onto the small diameter step section 12 formed on the other end portion of the hub main body 8 in the axial direction. Then, according to the on-fitting work, rolling contact surfaces of the rolling elements 5 and 5 (at the other side row in the axial direction) held by the holder 29b at the other side in the axial direction are made to abut the outer ring track 7b at the other side in the axial direction and formed on an inner circumferential surface of a portion close to the other end of the outer ring 3 in the axial direction. Next, the cylindrical section 19 formed on the other end portion of the hub main body 8 in the axial direction is plastically deformed outward in the radial direction to form the caulking section 20. Then, the inner ring 9 is fixed to the hub main body 8 by pressing in the axial direction the other end surface of the inner ring 9 in the axial direction with the caulking section 20.
Further, as shown in FIG. 6, the hub-side face spline 21 is formed on the other end surface of the caulking section 20 in the axial direction by performing rotary forging using a roll 30 having a central axis β inclined with respect to a central axis (a central axis of the wheel-supporting rolling bearing unit 1) α of the hub main body 8 by a predetermined angle θ.
Further, in FIG. 6, members except for the hub main body 8 (the outer ring 3, the rolling elements 5 and 5, the inner ring 9, and so on) among members that constitute the wheel-supporting rolling bearing unit 1 are omitted. A tip surface (a lower end surface of FIG. 6) of the roll 30 forms a processing surface 31 in which a concave section 34 and convex sections 33 and 33 (see FIG. 9) are alternately disposed throughout the circumference thereof. In a state in which the processing surface 31 of the roll 30 is pressed toward the other end surface of the caulking section 20 in the axial direction, the roll 30 is rotated about the central axis α of the hub main body 8.
Here, the roll 30 is rotatably supported about a central axis β thereof. Accordingly, in a state before the hub-side face spline 21 (i.e., a concavo-convex section in the circumferential direction that will serve as the hub-side face spline 21) is formed on the other end surface of the caulking section 20 in the axial direction, when the roll 30 is rotated about the central axis α of the hub main body 8, the roll 30 is rotated self-rotation) about the central axis β thereof on the basis of frictional engagement between tip surfaces of the convex sections 33 and 33 formed on the processing surface 31 and the other end surface of the caulking section 20 in the axial direction. On the other hand, after the hub-side face spline 21 is formed to a certain extent (i.e., a tooth depth of the hub-side face spline 21 is large to a certain extent), when the roll 30 is rotated about the central axis α of the hub main body 8, the roll 30 self-rotates on the basis of engagement (meshing) between the concave section 34 and the convex sections 33 and 33 that constitute the processing surface 31 and the hub-side face spline 21. According to this configuration, as the caulking section 20 is plastically deformed by pressing the processing surface 31 of the roll 30 toward the other end surface of the caulking section 20 in the axial direction, the hub-side face spline 21 is formed on the other end surface of the caulking section 20 in the axial direction, and further, a tooth depth of the hub-side face spline 21 is increased to complete the processing.
A method for manufacturing the above-mentioned wheel-supporting rolling bearing unit 1 has room for improvement from an aspect of suppressing manufacturing cost while securing durability of the wheel-supporting rolling bearing unit 1. That is, work of forming the hub-side face spline 21 (rotary forging) is performed by rotating the roll 30 about the central axis α of the hub main body 8 in a state in which the processing surface 31 of the roll 30 is pressed against the other end surface of the caulking section 20 in the axial direction with a large force (pressing force) F.
During such rotary forging, when an inclined angle θ of the central axis β of the roll 30 with respect to the central axis α of the hub main body 8 is small, an abutting area between the processing surface 31 of the roll 30 and the other end surface of the caulking section 20 in the axial direction, i.e., an abutting area between the tip surfaces of the convex sections 33 and 33 that constitute the processing surface 31 and the other end surface of the caulking section 20 in the axial direction, is increased.
As a result, a load (i.e., an abutting surface pressure between the tip surfaces of the convex sections 33 and 33 and the other end surface of the caulking section 20 in the axial direction) applied from the tip surfaces of the convex sections 33 and 33 to the other end surface of the caulking section 20 in the axial direction (i.e., a portion of the other end surface of the caulking section 20 in the axial direction that will serve as a tooth bottom of the hub-side face spline 21) decreases.
Accordingly, when the inclined angle θ is small, a load applied from the tip surfaces of the convex sections 33 and 33 to the other end surface of the caulking section 20 in the axial direction should be secured by increasing the pressing force F.
FIG. 7 shows a relation between the inclined angle θ and a maximum value of the pressing force F. As apparent from FIG. 7, when the inclined angle θ is 5°, the pressing force should have a value that is twice that when the inclined angle θ is 15°. When the pressing force F is increased, a processing apparatus used for the rotary forging is increased in size, and thus manufacturing cost is increased.
On the other hand, when the inclined angle θ is increased, straightness of a cross-sectional shape of a tooth surface of a tooth 32 (a side surface of a tooth) that constitutes the hub-side face spline 21 is decreased. Reasons for this will be described below. When the number of convex sections 33 and 33 that constitute the processing surface 31 of the roll 30 is set equal to the number of teeth 32 that constitute the hub-side face spline 21, and it is assumed that, when rotary forging is performed on the other end surface of the caulking section 20 in the axial direction, a sliding in the circumferential direction of the engaging section between the processing surface 31 and the other end surface of the caulking section 20 in the axial direction does not occur, arbitrary points P on the processing surface 31 of the roll 30 draw a closed loop shaped trajectory. The trajectory drawn by the points P during the rotary forging will be described with reference to FIG. 6.
First, it is assumed that the point P is on an engaging section (a point P0 in FIG. 6) with the other end surface of the caulking section 20 in the axial direction. Coordinates P0(x1, y1, z1) of the point P0 are represented by the following equation (1) in a rotation coordinate system (x1, y1, z1) in which an origin is set at an intersection between the central axis α of the hub main body 8 and the central axis β of the roll 30, and among an x1 axis, a y1 axis and a z1 axis that are perpendicular to each other, the y1 axis coincides with a front-rear direction of FIG. 6 and the z1 axis coincides with the central axis β of the roll 30.
                              [                      Math            .                                                  ⁢            1                    ]                ⁢                                                                                                            P            0                    ⁡                      (                                          x                1                            ,                              y                1                            ,                              z                1                                      )                          =                  [                                                                      L                  ⁢                                                                          ⁢                  cos                  ⁢                                                                          ⁢                  θ                                                                                    0                                                                                                          L                    ⁢                                                                                  ⁢                    sin                    ⁢                                                                                  ⁢                    θ                                    -                  d                                                              ]                                    (        1        )            
Further, θ in the equation (1) represents an inclined angle of the central axis β of the roll 30 with respect to the central axis α of the hub main body 8 (an angle formed between the central axis α and the central axis β), d represents a distance between an intersection between the central axis α and the central axis β and in intersection between the central axis β and the other end surface of the caulking section 20 in the axial direction (a distance in the axial direction of the central axis β), and L represents a distance between the point P0 and an intersection between the central axis β and the other end surface of the caulking section 20 in the axial direction (a distance in the radial direction).
During the rotary forging, the roll 30 is rotated (self-rotation) about the central axis β (the z1 axis) thereof. Coordinates PA(x1, y1, z1) of a point PA in the rotation coordinate system obtained by rotating the point P0 by an angle λ around the z1 axis are represented as the following equation (2).
                              [                      Math            .                                                  ⁢            2                    ]                ⁢                                                                                                            P            A                    ⁡                      (                                          x                1                            ,                              y                1                            ,                              z                1                                      )                          =                                            [                                                                                          cos                      ⁢                                                                                          ⁢                      λ                                                                                                                          -                        sin                                            ⁢                                                                                          ⁢                      λ                                                                            0                                                                                                              sin                      ⁢                                                                                          ⁢                      λ                                                                                                  cos                      ⁢                                                                                          ⁢                      λ                                                                            0                                                                                        0                                                        0                                                        1                                                              ]                        ⁡                          [                                                                                          L                      ⁢                                                                                          ⁢                      cos                      ⁢                                                                                          ⁢                      θ                                                                                                            0                                                                                                                                      L                        ⁢                                                                                                  ⁢                        sin                        ⁢                                                                                                  ⁢                        θ                                            -                      d                                                                                  ]                                =                      [                                                                                L                    ⁢                                                                                  ⁢                    cos                    ⁢                                                                                  ⁢                    λ                    ⁢                                                                                  ⁢                    cos                    ⁢                                                                                  ⁢                    θ                                                                                                                    L                    ⁢                                                                                  ⁢                    sin                    ⁢                                                                                  ⁢                    λ                    ⁢                                                                                  ⁢                    cos                    ⁢                                                                                  ⁢                    θ                                                                                                                                          L                      ⁢                                                                                          ⁢                      sin                      ⁢                                                                                          ⁢                      θ                                        -                    d                                                                        ]                                              (        2        )            
The coordinates PA(x1, y1, z1) of the point PA in the rotation coordinate system are represented (converted) with a rotation coordinate system (x, y, z) in which an origin is set at an intersection between the central axis α and the central axis β, and among an x axis, a y axis and a z axis that are perpendicular to each other, the y axis is set to coincide with the front-rear direction of FIG. 6 and the z axis is set to coincide with the central axis α of the hub main body 8, to yield the following equation (3).
                              [                      Math            .                                                  ⁢            3                    ]                ⁢                                                                                                            P            A                    ⁡                      (                          x              ,              y              ,              z                        )                          =                                            [                                                                                          cos                      ⁢                                                                                          ⁢                      θ                                                                            0                                                                              sin                      ⁢                                                                                          ⁢                      θ                                                                                                            0                                                        1                                                        0                                                                                                                                      -                        sin                                            ⁢                                                                                          ⁢                      θ                                                                            0                                                                              cos                      ⁢                                                                                          ⁢                      θ                                                                                  ]                        ⁡                          [                                                                                          L                      ⁢                                                                                          ⁢                      cos                      ⁢                                                                                          ⁢                      λ                      ⁢                                                                                          ⁢                      cos                      ⁢                                                                                          ⁢                      θ                                                                                                                                  L                      ⁢                                                                                          ⁢                      sin                      ⁢                                                                                          ⁢                      λ                      ⁢                                                                                          ⁢                      cos                      ⁢                                                                                          ⁢                      θ                                                                                                                                                          L                        ⁢                                                                                                  ⁢                        sin                        ⁢                                                                                                  ⁢                        θ                                            -                      d                                                                                  ]                                =                                                 [                                                                                                                  L                        ⁢                                                                                                  ⁢                        cos                        ⁢                                                                                                  ⁢                        λ                        ⁢                                                                                                  ⁢                                                                              cos                            ⁢                                                                                                                                          2                                                ⁢                        θ                                            +                                              L                        ⁢                                                                                                  ⁢                                                  sin                          2                                                ⁢                        θ                                            -                                              d                        ⁢                                                                                                  ⁢                        sin                        ⁢                                                                                                  ⁢                        θ                                                                                                                                                        L                      ⁢                                                                                          ⁢                      sin                      ⁢                                                                                          ⁢                      λ                      ⁢                                                                                          ⁢                      cos                      ⁢                                                                                          ⁢                      θ                                                                                                                                                                                    -                          L                                                ⁢                                                                                                  ⁢                        cos                        ⁢                                                                                                  ⁢                        λ                        ⁢                                                                                                  ⁢                        cos                        ⁢                                                                                                  ⁢                        θsin                        ⁢                                                                                                  ⁢                        θ                                            +                                              L                        ⁢                                                                                                  ⁢                        sin                        ⁢                                                                                                  ⁢                        θ                        ⁢                                                                                                  ⁢                        cos                        ⁢                                                                                                  ⁢                        θ                                            -                                              d                        ⁢                                                                                                  ⁢                        cos                        ⁢                                                                                                  ⁢                        θ                                                                                                        ]                                                          (        3        )            
Here, the right side of the equation (3) is replaced with (X, Y, Z).
Meanwhile, during the rotary forging, the roll 30 is rotated about the central axis α of the hub main body 8. A rotation angle of the roll 30 about the central axis α (the z axis) when the roll 30 is rotated about the central axis β (the z1 axis) thereof by an angle λ is assumed to be ϕ. Coordinates PB(x, y, z) of a point PB in the rotation coordinate system obtained by rotating the point PA around the z axis by an angle ϕ are represented as the following equation (4).
                              [                      Math            .                                                  ⁢            4                    ]                ⁢                                                                                                            P            B                    ⁡                      (                          x              ,              y              ,              z                        )                          =                                            [                                                                                          cos                      ⁢                                                                                          ⁢                      ϕ                                                                                                                          -                        sin                                            ⁢                                                                                          ⁢                      ϕ                                                                            0                                                                                                              sin                      ⁢                                                                                          ⁢                      ϕ                                                                                                  cos                      ⁢                                                                                          ⁢                      ϕ                                                                            0                                                                                        0                                                        0                                                        1                                                              ]                        ⁡                          [                                                                    X                                                                                        Y                                                                                        Z                                                              ]                                =                      [                                                                                                      X                      ⁢                                                                                          ⁢                      cos                      ⁢                                                                                          ⁢                      ϕ                                        -                                          Y                      ⁢                                                                                          ⁢                      sin                      ⁢                                                                                          ⁢                      ϕ                                                                                                                                                              X                      ⁢                                                                                          ⁢                      sin                      ⁢                                                                                          ⁢                      ϕ                                        +                                          Y                      ⁢                                                                                          ⁢                      cos                      ⁢                                                                                          ⁢                      ϕ                                                                                                                    Z                                                      ]                                              (        4        )            
Here, provided that sliding in the circumferential direction is not generated in the engaging section between the processing surface 31 and the other end surface of the caulking section 20 in the axial direction, a relation between a rotation angle ϕ of the roll 30 about the central axis α of the hub main body 8 and a rotation angle λ of the roll 30 about the central axis β thereof is represented as the following equation (5).
                              [                      Math            .                                                  ⁢            5                    ]                ⁢                                                                                                -          ϕ                =                                            L              ⁢                                                          ⁢              cos              ⁢                                                          ⁢              θ                        R                    ·          λ                                    (        5        )            
Further, R in the equation (5) represents a distance (a distance in the radial direction) between the point P0 and the central axis α.
When the equation (5) is substituted into the equation (4) and the rotation angle ϕ of the roll 30 about the central axis α is varied from 0 to 360 degrees, during the rotary forging, a trajectory T drawn by the point P on the processing surface 31 of the roll 30a can be obtained.
Specifically, the trajectory T drawn by the point P in the y-z plane (when seen in a direction of the x axis) has, for example, a reversed teardrop shape shown in FIG. 8. A trajectory T5 in FIG. 8 represents a trajectory drawn by the point P when the inclined angle θ is 5°, the distance R is 21.31 [mm], the distance L is 21.39 [mm], and the distance d is 0.93 [mm]. On the other hand, the trajectory T15 in FIG. 8 represents a trajectory drawn by point P when the inclined angle θ is 15°, the distance R is 21.31 [mm], the distance L is 22.06 [mm], and the distance d is 2.9 [mm].
As apparent from FIG. 8, when the inclined angle θ is increased, the trajectory T drawn by the point P (an area of a portion surrounded by the trajectory T) is increased (in particular, a width in a y direction is increased). Here, when the width of the trajectory T in the y direction is increased, displacement of the point P in the y direction according to rotation of the roll 30 about the central axis α is increased, and straightness of a cross-sectional shape of the tooth surfaces of the teeth 32 that constitute the hub-side face spline 21 is decreased.
That is, in the case of the method for manufacturing the wheel-supporting rolling bearing unit 1 in the related art, in a state in which a central position of the one concave section 34 (a central position of the roll 30 in the circumferential direction) of the plurality of concave sections 34 that constitute the processing surface 31 of the roll 30 is pressed toward the other end surface of the caulking section 20 in the axial direction (a state in which a virtual line γ parallel to the central axis β of the roll 30 and passes through a central position of the one concave section 34 is present in a virtual plane including the central axis α of the hub main body 8 and the central axis β thereof), as shown in FIG. 9(C), the tooth surface of the tooth 32 formed by the one concave section 34 (i.e., a side surface of the convex section in the circumferential direction that will serve as the tooth 32) is parallel to a side surface of the one concave section 34 in the circumferential direction when it abuts the tooth surface of the tooth 32.
When the roll 30 is rotated (revolved) about the central axis α from this state, the side surface of the one concave section 34 in the circumferential direction is inclined with respect to the tooth surface of the tooth 32 formed by the one concave section 34 (i.e., a side surface of the convex section in the circumferential direction that will serve as the tooth 32). An inclined angle of the side surface of the one concave section 34 in the circumferential direction with respect to the tooth surface of the tooth 32 is maximally increased in a state in which a central position of one convex section 33 in the plurality of convex sections 33 and 33 that constitute the processing surface 31 of the roll 30 is pressed toward the other end surface of the caulking section 20 in the axial direction {a state shown in FIG. 9(B) or 9(D)}. When the inclined angle is increased, the side surface of the one concave section 34 in the circumferential direction abuts the tooth surface of the tooth 32 locally, and the tooth surface of the tooth 32 is locally plastically deformed.
FIG. 10 is a schematic view showing a face spline tooth formed by rotary forging, FIG. 10(A) is a schematic view when an inclined angle of the central axis of the roll with respect to the central axis of the hub main body is 5°, and FIG. 10(B) is a schematic view when an inclined angle of the central axis of the roll with respect to the central axis of the hub main body is 15°. Like the portion surrounded by a chain line γ of FIG. 10(B), a radius of curvature of a cross-sectional shape of the tooth surface of the tooth 32 is reduced, and like the portion surrounded by a chain line ε, a step portion 35 is formed, and straightness of the cross-sectional shape of the tooth surface of the tooth 32 is decreased. When the straightness is decreased, in a state in which the hub-side face spline 21 is meshed with the joint-side face spline 26 formed on the outer ring 2 for a constant velocity joint (see FIG. 5), an abutting area between the tooth surfaces of the teeth 32 that constitute the hub-side face spline 21 and the teeth that constitute the joint-side face spline 26 is reduced, and it may be difficult to secure durability of the meshing section between the hub-side face spline 21 and the joint-side face spline 26, and therefore durability of the entire wheel-supporting rolling bearing unit 1.